In a typical nuclear reactor system, and in particular a pressurized water reactor (PWR) which is well known in the art, the reactor vessel includes a removable upper head. Such a design provides sufficient access to the interior space of the reactor vessel for such purposes as refueling of the reactor core, as well as various other maintenance procedures. The vessel head is secured to the reactor vessel by a plurality of stud bolts and nuts installed around the circumference of the vessel head flange and cooperating with a reactor vessel flange. Since the reactor vessel is generally filled with a coolant, typically water, a leak-tight seal is provided between the cooperating surfaces of the vessel closure head flange and the reactor vessel flange.
In order to effectuate this seal, an exemplary design of a vessel closure head includes two hollow, metallic O-rings or gaskets situated within concentric grooves in the bottom surface of the closure head flange. These O-rings, or gaskets, typically are circular-type metal tubes which contain the internal pressure, as in a PWR, in cooperation with the bolted joint between the closure head and the reactor vessel. The outside diameter of the O-rings is generally larger than the depth of the groove within the bottom surface of the closure head flange. In this manner, when the head is bolted to the vessel flange, the O-rings are somewhat flattened within the respective grooves by the relatively smooth and flat surface of the reactor vessel flange. Also, the O-rings are self-energized by permitting the internal pressure of the PWR to act on the inside of the O-rings through a plurality of slots within the O-ring. Thus, the double O-ring seal and flange design prevents leakage past the pair of O-rings; via the O-rings effectively closing off any spaces which may be present between the cooperating surfaces of the respective flanges.
To attach the O-rings to the bottom surface of the vessel head flange, a plurality of clips are used. These clips are inserted through some of the slots within the O-rings. Concentric with each of the grooves are, typically, threaded holes, one set for each groove and O-ring, within which are threaded bolts to secure the clip and thus the O-ring to the vessel head flange. Whenever the reactor vessel is opened by removal of the closure head, such as for refueling, the O-rings are replaced. Because the O-rings are retained in the reactor vessel head by clips so that they will be removed with the head, they can be replaced outside the refueling cavity. Nonetheless, the undersurface of the vessel closure head defines an irradiated environment which may be hazardous to maintenance personnel. For this reason, it is desirable that the O-rings and clips be removable as quickly as possible. This will minimize the installation time and thus reduce the time that personnel must be present within an irradiated environment. Typically, sixteen clips are used for each O-ring to secure the O-ring to the vessel head flange. This means that thirty-two bolts and clips would need to be unscrewed, changed and installed again each time the reactor vessel is opened.
One way to minimize the installation and removal time required for the O-rings is to substitute springs for the bolts used to secure the clips to the vessel head. (An example of such is shown in FIG. 10.) One end of the, spring is inserted into the slot within the O-ring, while the other is inserted within the threaded hole. The force of friction between the spring and the threaded hole is intended to secure the spring within the hole and prevent slippage of the O-ring and spring from the bottom surface of the vessel head flange. When the vessel head is lifted out of engagement with the reactor vessel flange, gravity acts on the O-ring and pulls it downward. If the friction force between the spring clip and the threaded hole is not sufficient, the spring clips may have a tendency to slip from the holes and thereby become disengaged with the reactor vessel head flange and the O-ring. It is therefore desirable to have a means for securing the O-rings within the grooves of the reactor vessel closure head flange which will not become separated from the O-ring and/or the threaded hole.